(1) Technical Field
The present invention relates generally to communication systems. More particularly, the invention relates to communication systems for transmission of information signals between two exchange telephone devices over a transmission network and to a method of data compression/decompression using statistical criteria.
(2) Background Discussion
The telecommunication environment is in full evolution and has changed considerably these recent years. The principal reason has been the spectacular progress realized in the communication technology due to the maturing of fiber optical transmission (high-speed rates can now be sustained with very low bit error rates) and the universal use of digital technologies within private and public telecommunications networks.
In relation with these new emerging technologies, the offerings of the telecommunication companies, public or private, are evolving. Indeed, the emergence of high speed transmissions entails an explosion in the high bandwidth connectivity; the increase of the communication capacity generates more attractive tariffs; a higher flexibility is offered to the users to manage their growth through a wide range of connectivity options, an efficient bandwidth management and the support of new media; and once sampled and digitally encoded, voice, video and image derived data can be merged with pure data for a common and transparent transport.
In a first step, networks were primarily deployed with TDM (Time Division Multiplexing) technology to achieve cost savings through line aggregation. These systems easily supported the fixed bandwidth requirements of host/terminal computing and 64 Kbps PCM (Pulse Code Modulation) voice traffic.
The data transmission is now evolving with a specific focus on applications and by integrating a fundamental shift in the customer traffic profile. Driven by the growth of workstations, the local area networks (LAN) interconnection, the distributed processing between workstations and super computers, the new applications and the integration of various and often conflicting structuresxe2x80x94hierarchical versus peer to peer, wide (WAN) versus local (LAN) area networks, voice versus dataxe2x80x94the data profile has become higher in bandwidth, bursting, non deterministic and requires more connectivity. Based on the above, it is clear that there is strong requirement to support distributed computing applications across high speed backbones that may be carrying LAN traffic, voice, video, and traffic among channel attached hosts, business workstations, engineering workstations, terminals, and small to intermediate file servers. This traffic reflects a heterogeneous mix of: end user network protocols, and real time (steady stream traffic such as voice and video) and non real time (bursty nature traffic such as interactive data) transmissions.
The vision of a high-speed protocol-agile backbone network is the driver for the emergence of fast packet switching network architecture in which data, voice, and video information are digitally encoded, chopped into small packets and transmitted through a common set of nodes and links.
The basic advantage of packet switching techniques, compared to circuit switching techniques, is to allow a statistical multiplexing of different types of data over a line (data, voice, image, video), which optimizes the bandwidth. The drawback of packet switching techniques, compared to circuit switching techniques, is that packet switching typically introduces an overhead for every packet that is transported. One can reduce such an overhead by using compression techniques. However, if these techniques apply well to the compression of individual telephony circuits, they are not appropriate to the compression of complete telephony trunks between private or public automatic exchanges (PABX) and central exchanges (CX).
The existing techniques of compression depend upon the information which is transmitted that is voice, data or video. Several types of voice compression are available. They are generally adapted to perform compression on a limited number of channels as they require a lot of computing power to be efficient. They are more efficient if they work on a lot of samples introducing delay and complexity. Furthermore, they are not reversible, which means loss of information if the contents of the channel are not only voice signals but fax, modem, data or video signals.
For pure data, the compression protocol is principally V.42 bis based on Lempel-Ziv algorithm. In all cases, data compression is fully reversible as the purpose is of course to have the same data at both the input and the output, which is not the case for voice or video. However, the algorithm used for data compression takes advantage of the abnormal distribution of data which is not random so that the compression ratio which is applied to voice and video signal is not good. Furthermore, the data compression on high speed line requires an important computing power.
As far as compression of video signals is concerned, the techniques of compression (e.g., MPEG or JPEG) are very specific to image characteristics, require an important computing power and are not compatible with data or voice compression.
Accordingly, an object of the invention is to provide a method of compression specially oriented to compression of voice channels which present a very low delay and a full reversibility and is therefore compatible with all kinds of data transported over a transmission network.
The invention is achieved in a data transmission system and method for compressing data in the transmission system wherein multiplexed channels are transported over a transmission network of the type comprising a plurality of switching nodes interconnected by connection lines. The network includes intermediary switching nodes connected only to the other switching nodes and end switching nodes each connected to at least an exchange telephone device. Each switching node is associated with a control point in charge of controlling the exchange of data signals carried out by switching the channels in the network between two exchange telephone devices. The control points aiding the associated switching nodes determine the best route between any source exchange telephone device and any destination exchange telephone device. Each of the multiplexed channels transports data bytes representing the data signals from one source exchange telephone device to one destination exchange device during an exchange of information therebetween. The method comprises the steps of comparing, for each multiplexed channel, the signal value associated to each one of a plurality of n consecutive data bytes to a predetermined threshold; deleting, in case said signal value for all said n data bytes is less than the predetermined threshold, all bits which are not necessary to represent the signal value from each of the n data bytes; building a frame by concatenating either the n data bytes when they are not modified or the n modified data bytes when bits have been deleted therefrom; adding to the frame an identifier indicating whether the data bytes are modified or not before transmitting this frame over the transmission network; decompressing the frame by determining the identifier value indicating the composition of the bytes; removing the identifier from the bytes; loading the bytes into a buffer and transmitting the bytes to the destination exchange telephone device.